Presently, metal sheet stock, for example, such as electrotinplate, tin free steel (TFS), and aluminum are widely used for can stock after applying one or more coats of lacquer. However, the employ of such a lacquer coating has associated drawbacks, including increased energy costs due to extended curing times, and the discharge of solvent during curing which must be disposed, for example, by incineration to prevent environmental pollution.
To avoid such problems as mentioned above, the lamination of a thermoplastic polyester resin film on a metal sheet has recently been described, for example, in U.S. Pat. No. 4,517,255, Laid-Open Japanese Patent Application No. Hei 3-212433, U.S. Pat. No. 4,614,691, and Japanese Patent Publication Nos. Sho 57-23584 and Sho 60-4058.
The polyester resin film laminated metal sheet according to these references are formed to a can by the following can making methods:
(1) One side of the laminate which is used for outside of the can is coated with lacquer or printing ink and then heated for curing the precoated lacquer or printing ink, at a temperature of about 160 to 220.degree. C. for about 1 to about 20 minutes. After that, the laminate is formed to a can. PA1 (2) After the laminate is formed to a can, the outside of the can is coated with lacquer or printing ink and then heated for curing the precoated lacquer or printing ink at a temperature of about 160 to about 220.degree. C. for about 1 to about 20 minutes. PA1 (3) The laminate is formed to a can without coating and heating. PA1 (1) characteristics of the employed polyester resin film; PA1 (2) characteristics of the polyester resin film after laminating to the metal sheet, especially the characteristics of the polyester resin layer contacting the surface of the metal sheet; PA1 (3) use of the surface treated metal sheet which preferably has excellent adhesion to the laminated polyester resin film, and PA1 (4) selection of laminating conditions in response to the characteristics of the employed polyester resin film. PA1 "K" re presents a constant for crystallization speed of the employed polyester resin; PA1 "n" represents avrami index; PA1 "t" represents time (seconds); PA1 "Io" represents a transparent intensity of depolarized light at the starting point in the measurement, or more particularly Io represents a value which can be obtained after the melted polyester resin film is dropped into a silicon oil bath for crystallization and then kept for 10 seconds ; PA1 "It" represents a transparent intensity of depolarized light after t seconds in the measurement, or more particularly It represents the value after 10+t seconds; and PA1 "Ig" represents a transparent intensity of depolarized light at the end point in the measurement, or Ig represents the value in which I-log t curve shows almost a straight line. PA1 (1) relative ratio of the density in the laminated polyester resin film; and PA1 (2) residual degree of biaxial orientation in the laminated polyester resin film. PA1 Sample (a): A polyester resin film laminated metal sheet is immersed into dilute hydrochloric acid solution at 25.degree. C. After dissolution of the metal sheet, the obtained film is rinsed with water for 3 hours and then dried in a desiccator in the presence of silica gel (drying agent) for 1 day at 30.degree. C. PA1 Sample (b): The same polyester resin film laminated metal sheet as prepared in sample (a) is heated in a nitrogen atmosphere at a temperature of the melting temperature of said film +30.degree. C. for 1 minute and then immediately immersed into liquid nitrogen. After that, the only laminated film is obtained by using the same method as in the preparation of sample (a). PA1 Sample (c): The same polyester resin film laminated metal sheet as prepared in sample (a) is heated for 1 hour in a nitrogen atmosphere at a temperature where a maximum density is obtained within a range of crystallization temperature of the laminated polyester resin film. After that, a laminated film is obtained by using the same method as in the preparation of sample (a). PA1 (1) the X-ray diffraction intensity of the polyester resin film before and after lamination to the metal sheet is measured within the range of 2.THETA.=20.degree. to 30.degree.; PA1 (2) a point at 2.THETA.=20.degree. and 2.THETA.=30.degree. is connected by a straight line, and this line is designated as the base line; PA1 (3) a height of the peak appearing in 2.THETA.=23.degree. to 29.degree. of the diffraction intensity curve from the base line is measured; and the height in the polyester resin film before and after lamination to the metal sheet is represented by Ia and Ib, respectively; and PA1 (4) a residual degree of biaxial orientation (BO) is represented by the following equation: EQU BO (%)=Ib/Ia.times.100
The can obtained by each method (1)-(3) described hereinabove is treated by hot steam and hot water at 100 to 130.degree. C. in a retort for the sterilization of foods packed therein.
In particular, in order to neck the upper part of the formed deeply drawn can having high can height such as a drawn and stretch formed can, the formed can is reheated for the relief of the increased internal stress in the laminated polyester resin film in the upper part of said formed can at a temperature of about 180 to about 220.degree. C. for about 1 to about 4 minutes, because the internal stress in the laminated polyester resin film is accumulated in the formed part by the severe forming. If the aforesaid formed can is not reheated, the laminated polyester resin film is easily peeled off from the metal sheet by the necking of the upper side of the formed can.
Therefore, it is preferable that the polyester resin film laminated metal sheet used for a can which is formed by various methods described above is excellent in all characteristics inclusive of the formability, the adhesion of the polyester resin film to the metal sheet after severe forming and the appearance in the outside of the can after retort treatment. However, the laminate according to these references are not always excellent in all of the characteristics described above.
U.S. Pat. No. 4,517,255 describes a method for laminating a crystalline polyester resin film to a metal sheet heated to a temperature above the melting temperature of the polyester resin film, and thereafter immediately quenching the laminate. In this reference, the crystalline polyester resin film is said to be sufficiently adhered to the metal sheet by an amorphous non-oriented polyester resin layer which is formed at the interface of the crystalline polyester resin film and the metal sheet as a result of the heating step. However, when the laminate according to this reference is formed to a can by can making method (3) described above, and thereafter the formed can is treated in a retort for sterilization of foods packed therein, innumerable milky spots which deteriorate the commodity value of the packaging are observed on the outside of the can, although the laminated polyester resin film is not peeled off from the metal sheet. It is thought that such milky change is due to the difference of the recrystallization speed of the amorphous non-oriented polyester resin layer in the part contacting hot steam from that contacting hot water. If the surface of the polyester resin film laminated metal sheet is uniformly wetted by hot water or hot steam, the milky spots are not observed.
If the laminate according to this reference is formed to a can by the can making methods (1) or (2) described above, such milky change is not always observed in the outside of the can after retort treatment. However, a large spherulite of polyester resin grows by the recrystallization of the amorphous non-oriented polyester resin layer during heating the laminate or the obtained can for a long time. As the result, the laminated polyester resin film is easily peeled off from the metal sheet by severe forming.
Laid-Open Japanese Patent Application No. Hei 3-212433 describes a method for producing a copolyester film laminated metal sheet which is said to have excellent resistance to such milky change by a retort treatment. In particular, this reference is characterized by laminating a copolyester resin film consisting of 75 to 99 mole % of polyethylene terephthalate and 1 to 25 mole % of other polyester resin on a metal sheet at a temperature below the melting temperature and above the softening temperature of the copolyester resin film, and thereafter rapidly quenching the laminate in order to decrease the amount of the amorphous non-oriented copolyester resin layer formed as a result of the heating step during lamination, which causes the milky change by a retort treatment such as described above. In the method according to this reference, a small amount of the amorphous non-oriented polyester resin layer is inevitably formed between the surface of the metal sheet and the biaxially oriented copolyester resin layer. If the amorphous non-oriented copolyester resin layer is not formed at all, the laminated biaxially oriented copolyester resin film will not adhere to the metal sheet. Therefore, according to this reference, it is very difficult to obtain a polyester resin film laminated metal sheet which exhibits good adhesion and formability and does not exhibit a milky change during retort treatment, if the laminate is formed to a can by the can making method (3), because the copolyester resin melted at a temperature below the melting temperature and above the softening temperature of the employed copolyester resin film has high viscosity and the surface of the metal sheet is not uniformly wetted by the melted copolyester resin.
If the laminate according to this reference is formed to a can by the can making methods of (1) or (2), the laminated copolyester resin film in the formed part is easily peeled off from the metal sheet, because the adhesion of the laminated copolyester resin film is inferior to that set forth in U.S. Pat. No. 4,517,255.
It is possible experimentally to prevent the milky change of the laminate having an amorphous non-oriented polyester resin layer from occurring by the methods described in these patents set forth above by a use of special retort, wherein the laminate is usually in contact with hot steam or hot water only. However, these methods are not economical. Furthermore, this milky change may be also prevented by reheating the laminate before quenching at a temperature above the glass transition temperature for a long time, for example, at 160.degree. C. for 120 seconds, in the production process of the polyester resin film laminated metal sheet. However, this reheating method is not suitable for the continuous production of the polyester resin film laminated metal sheet at high speed, and is not suitable from the standpoint of economy, because an addition of reheating equipment is necessary. Furthermore, such milky change is also prevented by the reheating of the laminate in the can making methods (1) or (2). However, the adhesion of the laminated polyester resin film to the metal sheet becomes poor by severe forming.
In the laminate according to U.S. Pat. No. 4,614,691, such milky change by a retort treatment is not observed in the outside of the can formed by the can making method (1) or (2). However, this milky change is slightly observed in the outside of the can formed by the can making method (3), because the presence of an amorphous non-oriented polyester resin layer which causes such milky change is substantially reduced as a result of lamination at a temperature below the melting temperature of the employed polyester resin film. However, the use of a polyester resin film precoated with the specified adhesive is an absolute requirement in this reference. Therefore, the method in this reference is disadvantageous from the standpoint of the material cost and the treatment of a large volume of solvent discharged during curing the precoated adhesive which causes air pollution.
Japanese Patent Publication No. Sho 57-23584 describes a metal structure covered with a thermoplastic polyester resin produced by esterification of dicarboxylic acid, in which terephthalic acid is at least 45 mole % of said dicarboxylic acid with diol in which 1,4-butane diol is at least 55 mole % of said diol. The polyester resin has a relative viscosity of 1.2 to 1.8, a tack point of not lower than 130.degree. C., and a degree of crystallinity of up to 30%. In this reference a metal substrate is covered with a thermoplastic resin containing above 45 mole % of polybutylene terephthalate which has an amorphous non-oriented structure. Therefore, if the laminate according to this reference is formed to a can by the can making method (3) and the formed can is treated in a retort after packing foods, the outside of the can may become milky, because the laminated amorphous non-oriented layer is non-uniformly recrystallized by retort treatment. Further, when the laminate according to this reference is formed to a severe formed can such as a drawn and stretch formed can by the can making method (1) or (2), many cracks may arise in the severe formed part, because the biaxially oriented polyester resin having excellent formability is not present on the surface of the laminate according to this reference.
Additionally, Japanese Patent Publication No. Sho 60-4058 describes a can end produced by a polyester resin laminated metal sheet, which comprises heat bonding a polyester resin on a metal sheet, wherein the polyester resin is produced by esterification of dicarboxylic acid in which terephthalic acid is at least 66 mole % of said dicarboxylic acid with diol in which 1,4-butane diol is at least 45 mole % of said diol, and has an intrinsic viscosity of 0.7 to 2.8.
The can end according to this reference which is formed by the can making method (3) can not be used for the application in which excellent resistance to milky change by retort treatment is required, because an amorphous non-oriented structure is formed which is the same as that in Japanese Patent Publication No. Sho 57-23584. Further, if the can end according to this reference is obtained by the can making method (1) or (2), many cracks may arise in severe formed part such as the part double seamed to the can body.
As described above, the laminates according to these references do not have all of the characteristics required for the can which is treated in a retort after packing foods.
Accordingly, a primary objective of the present invention is to provide a polyester resin film laminated metal sheet which has improved resistance to such milky change as discussed above which is observed on the outside of a can after retort treatment for sterilization of the packed foods, and which also has improved adhesion of the laminated polyester resin film to a metal sheet and formability to can ends, can bodies in three piece cans, drawn and redrawn cans, drawn and stretch formed cans and screw caps.
It is another objective of this invention to provide an economical method for the continuous production of such a polyester resin film laminated metal sheet at high speed.